Hi Sam,

Have you considered using a Dynamic Audience?  There's a rule called 'Certification Status' which you can use to group together users that are currently certified.

If you combine this rule with the Audience Based Visibility option (you can turn this on via Advanced Features) then you can limit course access to your new Certified user Audience.

I think that might do the trick! Just remember to have users automatically added and removed from the audience when they fail to meet the criteria.

Thanks for your question and I hope this helps :)

Meredith